Conventionally, a cutting insert as described in Patent document 1 has been well known as a cutting tool for use in a cutting process of a workpiece. The cutting insert described in Patent document 1 is specifically used in a milling process, such as a face milling process or an end milling process. The cutting insert described in Patent document 1 includes an upper cutting edge disposed along an intersection of an upper surface and a side surface, and a lower cutting edge disposed along an intersection of a lower surface and the side surface. The cutting edges are respectively made up of a major cutting edge having a projecting curvilinear shape, and a minor cutting edge that is continuous with the major cutting edge and has a straight line shape in a top plan view.
In the case of including the major cutting edge and the minor cutting edge as described above, the cutting process of the workpiece is performed mainly by the major cutting edge. On this occasion, the minor cutting edge may not be used in the cutting process, but may be used as a flat cutting edge for the purpose of improving a finished surface.
When the cutting process is performed using the cutting insert described in Patent document 1, because the upper cutting edge and the lower cutting edge are included as the cutting edge, an axial rake and a radial rake of a holder are respectively set to negative values in order to avoid interference of the cutting edge on the opposite side during the cutting process. However, the cutting performance of the cutting edges may be deteriorated when the axial rake and the radial rake respectively have the negative values. It is therefore necessary to increase a rake angle. This leads to the problem that the cutting edge strength is lowered and consequently the durability of the cutting insert is lowered.
The present invention has been made in view of the above problem, and an object thereof is to provide a cutting insert having satisfactory durability while ensuring satisfactory cutting performance of the cutting edges, as well as a cutting tool and a method of producing a machined product.    Patent Document 1: Japanese Unexamined Patent Publication No. 2012-500732